1. Field of the Invention
The present invention pertains to a radiographic image erasing device and particularly relates to a radiographic image erasing device, a radiographic image erasing program, and a radiographic image reading and erasing device that erase a radiographic image recorded in a storage phosphor sheet.
2. Related Art
When radiation (X-rays, α rays, β rays, γ rays, ultraviolet rays, an electron beam, etc.) is applied to certain types of phosphors, some of the energy of this radiation is stored in the phosphors, and when excitation light such as visible light is applied to the phosphors thereafter, the phosphors exhibit photostimulated luminescence in response to the stored energy.
Phosphors that exhibit this property are called storage phosphors or photostimulable phosphors. To date, various storage phosphors have been known, and as representative examples, barium halide phosphors that are activated by a rare-earth element such as europium and oxyhalide phosphors that are activated by a rare-earth element such as cerium are known. Further, storage phosphors in which various additives have been introduced to those phosphors are also known.
A method that forms the storage phosphor into a sheet, either using a binder or not using a binder, to make a storage phosphor sheet, uses the storage phosphor sheet to record radiographic image information relating to the human body or the like, scans the storage phosphor sheet with excitation light to cause the storage phosphor sheet to be photostimulated and emit light, photoelectrically reads the photostimulated luminescence to obtain image signals, and next processes the image signals to obtain an image with good diagnostic suitability has been proposed as a radiographic image recording and reproducing method (e.g., Japanese Patent Application Laid-Open No. 55-12429 (patent document 1)). In this radiographic image recording and reproducing method, it is described that in order to separate the wavelength regions of the excitation light and the photostimulated luminescence light and efficiently detect the extremely weak photostimulated luminescence light, detecting photostimulated luminescence light of 300 to 500 nm with excitation light in the wavelength region of 600 to 700 nm is preferred, and for this reason a storage phosphor that emits photostimulated luminescence light of 300 to 500 nm when the storage phosphor is excited with light of 600 to 700 nm is preferably used. The storage phosphor sheet has many forms, such as a general sheet-like storage phosphor sheet, a storage phosphor sheet formed in the shape of a belt, or a storage phosphor sheet formed in the shape of a drum, but in the present specification these will be collectively called “sheets”.
Radiographic images that have been stored and recorded in the storage phosphor sheets can be erased, so the storage phosphor sheets have the advantage that they can be repeatedly used. Consequently, in radiographic image recording and reproducing methods, the storage phosphor sheet is generally repeatedly used. However, if excitation light of a sufficient intensity is applied at the time when the radiographic image that has been stored and recorded is read from the storage phosphor sheet, the stored radiation energy corresponding to the radiographic image information that had been recorded is released to the outside and should disappear, but in actuality the stored radiation energy cannot be completely erased by only the excitation light that is applied at the time of the reading. Consequently, when the storage phosphor sheet is repeatedly used, there is the problem that the radiographic image that was captured the previous time remains and becomes noise in the radiographic image that is formed the next time.
Further, miniscule amounts of radioactive isotopes such as Ra and K are mixed in the storage phosphor sheet, so because of radiation radiated from these radioactive isotopes, the storage phosphor sheet stores radiation energy even when left unattended, and this also causes noise. Moreover, radiation energy is stored in the storage phosphor sheet because of environmental radiation such as cosmic radiation and radiation from radioactive isotopes in the environment. The radiation energy (called “fog”) that is stored while the storage phosphor sheet is left unattended also becomes noise with respect to the radiographic image that is captured the next time, so this fog must also be erased before the next imaging.
In the above-described radiographic image recording and reproducing method that repeatedly cyclically uses the storage phosphor sheet, in order to prevent the occurrence of noise resulting from the radiographic image that was captured the previous time and which remains in the storage phosphor sheet and noise resulting from fog, performing an operation that applies light of a wavelength including light in the excitation light wavelength region of the storage phosphor to the storage phosphor sheet to sufficiently release the remaining radiation energy and erase the residual radiographic image before recording new radiographic image information in the storage phosphor sheet is already known.
As the erasing methods, a method that uses a light source that emits light of a relatively long wavelength, such as a tungsten lamp, a halogen lamp, or an infrared lamp, which radiate visible light to infrared light, a method that uses light of a relatively short wavelength of 400 to 600 nm emitted by a fluorescent lamp, a laser light source, a sodium lamp, a neon lamp, a metal halide lamp, or a xenon lamp, and a method that performs erasure one time on the storage phosphor sheet and, immediately before reuse of the storage phosphor sheet, performs erasure a second time with an application amount that is ⅕ to 3/10,000 in comparison to the application amount in the first erasure are known. Additionally, it is regarded that erasure particularly in the visible light region is efficient.
However, when erasure is performed with an erase light source that does not at all include a wavelength in the ultraviolet (UV) region, the residual image resulting from trapped electrons whose level is relatively deep and which are difficult to erase with visible light cannot be sufficiently erased. On the other hand, when erasure is performed with erase light that includes many wavelengths in the UV region, the residual image resulting from the deep trapped electrons can be erased, but new trapped electrons end up being formed by the erase light of the wavelengths in the UV region, and the residual image cannot be completely erased.
Consequently, it is extremely difficult to simultaneously erase images resulting from ordinary traps and deep traps and perform efficient and complete erasure, and the reality is that subtle control of the short-wavelength component in the erase light is needed because the effect of the residual image ends up appearing when attempting to subsequently perform high-sensitivity imaging. Therefore, a radiographic image erasing method has been proposed which can efficiently erase the residual image resulting from deep traps in addition to the image resulting from ordinary traps by performing erasure with erase light including a wavelength component in the UV region and thereafter performing erasure with erase light of a longer wavelength than the UV region (e.g., JP-A No. 5-119412 (patent document 2)).
However, the invention described in patent document 2 has a configuration that applies the erase light of the long-wavelength component using a filter that cuts the short-wavelength component, so it has the problem that erasure efficiency becomes worse because of light loss caused by transmitting the erase light through the filter.
Further, there are cases where, in order to reliably erase the radiographic image recorded in the storage phosphor sheet, the user would like to performance erasure once and thereafter reverse the conveyance direction of the storage phosphor sheet to perform erasure again.
In this case, as shown in FIG. 14A for example, the device is given a configuration in which short-wavelength light sources 100A and 100B are placed on both sides of a long-wavelength light source 102. In a case where the conveyance direction of a storage phosphor sheet P is the direction of arrow A in the drawing, the short-wavelength light source 100B and the long-wavelength light source 102 are switched on. As shown in FIG. 14B, in a case where the conveyance direction of the storage phosphor sheet P is the direction of arrow B, which is the opposite of the direction of arrow A, the short-wavelength light source 100A and the long-wavelength light source 102 are switched on.
However, in this case, it is necessary to dispose two short-wavelength light sources, but because one of those short-wavelength light sources is not used, there is the problem that light utilization efficiency is poor.
Further, as shown in FIG. 15A for example, in a case where the conveyance direction of the storage phosphor sheet P is the direction of arrow A in the drawing, it is necessary to place a short-wavelength light source 100 on the right side in the drawing and place a long-wavelength light source 102 on the left side. As shown in FIG. 15B, in a case where the conveyance direction of the storage phosphor sheet P is the direction of arrow B, which is the opposite of the direction of arrow A, it is necessary to place the short-wavelength light source 100 on the left side in the drawing and place the long-wavelength light source 102 on the right side.
However, in this case, there is the problem that the thickness of the device ends up increasing because it is necessary to place and interchange the short-wavelength light source 100 and the long-wavelength light source 102 in two stages in the height direction with respect to the storage phosphor sheet P.